Back light illumination is typically used to detect defects in non-transparent parts of transparent wafers, e.g., finger cuts in LED wafers.
FIG. 1 shows a schematic representation of a typical system 1 for generating back light inspection of transparent or semitransparent objects 2 according to a first concept. Light source 6 is part of object carrier 10 for the transparent or semitransparent object 2. Light source 6 is covered with transparent plate 11 so that light 5 from light source 6 reaches object 2. Only a portion of light 7 of light source 6 reaches optical unit 8. From there, a portion of light 7 reaches sensor 12. Light source 6 is mounted on stage 9, which is able to position object 2 with respect to optical arrangement 8. The size of light source 6 needs to be at least equal to the size of the transparent or semitransparent object 2.
Due to heat dissipation and power requirements, the output of light source 6 and the inspection speed is limited. Since the size of the transparent or semitransparent objects 2 increases, the heat dissipation of light source 6 increases too. Furthermore, as shown in FIG. 1, the illumination is insufficient, since most of light 5 is not captured by optical arrangement 8.
Another problem is that a part of the transparent or semitransparent object 2 is illuminated with a different part of light source 6. This will result in different image brightness for different parts of object 2. Further, the movement of light source 6 and the transparent or semitransparent object 2 is limited due to the cabling (not shown) of the light source 6.
A schematic representation of another typical arrangement for back light inspection is shown in FIG. 2. Here, light source 6 is not part of object carrier 10. In this case, light source 6 for back light illumination is fixed light source 6. The transparent or semitransparent object 2 moves between light source 6 for back light illumination and optical unit 8. Stage 9 and carrier 10 need to be designed in such a way that that movement is not blocked by fixed light source 6. The size of the light source 6 needs to be at least equal to the size of the largest field of view of the optical unit 8.
The design of stage 9 is complex in order to enable movement of carrier 10 and the transparent or semitransparent object 2 around light source 6. Carrier 10 object 2 cannot be supported at the center since this space is taken by light source 6. The associated bending of carrier 10 and object 2 will lead to different focus positions at different locations on the transparent or semitransparent object 2 and image distortion in general.